reg + reference
in [Design]
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From: John Larkin on 27 Mar 2010 12:58 On Fri, 26 Mar 2010 13:43:32 -0800, "Artemus" <bogus(a)invalid.org> wrote: > >"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >news:13rpq5lqd1lj1k9flb9orfr2vcl2tuucpi(a)4ax.com... >> >> ftp://jjlarkin.lmi.net/V220_reg.JPG >> >> >> John >> > >How much current can that 1117 source from it's ADJ pin? >Art > Oh, I see what you're getting at, fig 4 ftp://jjlarkin.lmi.net/ARM_power.JPG if I catch your drift. Nice. John
From: John Larkin on 27 Mar 2010 13:16 On Sat, 27 Mar 2010 14:21:15 GMT, nico(a)puntnl.niks (Nico Coesel) wrote: >John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid> >>wrote: >> >>>John Larkin wrote: >>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>> >>> >>>Just curious, why not replace all this with one of these: >>> >>>http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf >>> >> >>>Much less space, cheaper. Unless you need the 3.3V somewhere. >> >>Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe. > >AFAIK the LPC1768 can run on lower voltages as well like down to 2.6V. >Not at full speed ofcourse. 40mA may be too optimistic though. We figure on sleeping at least half the time in operating mode. I'm using a 1-watt unregulated Murata sip DC-DC converter to power a channel, 12 volts in and +-15 out. The +-15 becomes floating ground, +15 to power the ARM, and +30 for the 4-20 mA current loop. So I have enough power to run the ARM full-blast during initialization, but not enough to output 20 mA and run the ARM at max. I would perfer to not add a switcher to make 3.3, or to add a lot of parts of any sort. We don't have time for custom magnetics: concept-to-shipping target is 60 days. Luckily, VME does have a lot of air flow. > >>The ADR could run off the +15, so maybe that's a better way to go. It >>would cost a little more, but the TC would be a lot better. >> >>Maybe I'll do that, the LM1117 for power and the ADR for the >>reference. >> >>Wait! LM4120 will work. +3 ref with 120 mV dropout! > >Be sure to decouple the reference properly. The ADC sinks quite some >current from the reference pin when sampling. The LM4120 allows me a maximum of 0.047 uF! The NXP documentation for this chip, around 1K pages of it, is horrible. We can find no mention of how much current the Vref pin uses. There's also a pin called, if I recall, Vcc(Reg)(3V3) whose function is unknown. Got any idea? Kyle must not either, since on their eval board they run it to a jumper header, making it our problem. The NXP documentation has roughly 1e6 syntactically-challenged words but not a single application circuit. They should have the schematic and code for a minimal LED blinker. Mixed-signal datasheets come in three sorts: Terrible analog specs Terrible digital specs Terrible specs. John
From: Joerg on 27 Mar 2010 13:33 John Larkin wrote: > On Sat, 27 Mar 2010 14:21:15 GMT, nico(a)puntnl.niks (Nico Coesel) > wrote: > >> John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid> >>> wrote: >>> >>>> John Larkin wrote: >>>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>>> >>>> Just curious, why not replace all this with one of these: >>>> >>>> http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf >>>> >>>> Much less space, cheaper. Unless you need the 3.3V somewhere. >>> Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe. >> AFAIK the LPC1768 can run on lower voltages as well like down to 2.6V. >> Not at full speed ofcourse. 40mA may be too optimistic though. > > We figure on sleeping at least half the time in operating mode. I'm > using a 1-watt unregulated Murata sip DC-DC converter to power a > channel, 12 volts in and +-15 out. The +-15 becomes floating ground, > +15 to power the ARM, and +30 for the 4-20 mA current loop. So I have > enough power to run the ARM full-blast during initialization, but not > enough to output 20 mA and run the ARM at max. I would perfer to not > add a switcher to make 3.3, or to add a lot of parts of any sort. We > don't have time for custom magnetics: concept-to-shipping target is 60 > days. > Doesn't have to always be custom. I can pay off to check what's in stock among these: http://www.coilcraft.com/lpr4012.cfm > Luckily, VME does have a lot of air flow. > >>> The ADR could run off the +15, so maybe that's a better way to go. It >>> would cost a little more, but the TC would be a lot better. >>> >>> Maybe I'll do that, the LM1117 for power and the ADR for the >>> reference. >>> >>> Wait! LM4120 will work. +3 ref with 120 mV dropout! >> Be sure to decouple the reference properly. The ADC sinks quite some >> current from the reference pin when sampling. > > The LM4120 allows me a maximum of 0.047 uF! > Doubtful that this is going to cut it. An option may be to run an RC filter with the R being just large enough to fulfill the LDO's stability criteria (might require a trip to a psychic reader to find out what those will really be ...). Then maybe, just maybe, you can place a nice big MLCC. > The NXP documentation for this chip, around 1K pages of it, is > horrible. We can find no mention of how much current the Vref pin > uses. There's also a pin called, if I recall, Vcc(Reg)(3V3) whose > function is unknown. Got any idea? Kyle must not either, since on > their eval board they run it to a jumper header, making it our > problem. > That warrants an inquiry to app engineering. I'd also would want to find out about spikes because if sort of irregular they can mess with the accuracy of your ADC channels. Since you go into sleep mode the surf on that rail will most likely be low frequency. > The NXP documentation has roughly 1e6 syntactically-challenged words > but not a single application circuit. They should have the schematic > and code for a minimal LED blinker. > Often they are written by non-native English speakers. Japanese datasheet can be rather interesting there. It's usually ok if you get the impression that they knew what they were doing. > Mixed-signal datasheets come in three sorts: > > Terrible analog specs > > Terrible digital specs > > Terrible specs. > No surprise these days :-( -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 27 Mar 2010 13:34 On Sat, 27 Mar 2010 08:47:41 -0700, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> On Fri, 26 Mar 2010 17:22:36 -0700, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> John Larkin wrote: >>>> On Fri, 26 Mar 2010 15:36:09 -0700, John Larkin >>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>> On Fri, 26 Mar 2010 14:34:19 -0700, Joerg <invalid(a)invalid.invalid> >>>>> wrote: >>>>> >>>>>> John Larkin wrote: >>>>>>> ftp://jjlarkin.lmi.net/V220_reg.JPG >>>>>>> >>>>>> Just curious, why not replace all this with one of these: >>>>>> >>>>>> http://www.analog.com/static/imported-files/Data_Sheets/ADR01_02_03_06.pdf >>>>>> >>>>>> Much less space, cheaper. Unless you need the 3.3V somewhere. >>>>> Yes, I need 3.3 to run an ARM processor, 40 mA or so maybe. >>>>> >>>>> The ADR could run off the +15, so maybe that's a better way to go. It >>>>> would cost a little more, but the TC would be a lot better. >>>>> >>>>> Maybe I'll do that, the LM1117 for power and the ADR for the >>>>> reference. >>>>> >>>>> Wait! LM4120 will work. +3 ref with 120 mV dropout! >>>>> >>>>> >>>>> John >>>> And of course, the LM4120 is a stability land-mine of its own. >>>> >>>> ThestandardapplicationcircuitfortheLM4120isshownin >>>> Figure1.Itisdesignedtobestablewithceramicoutput >>>> capacitorsintherangeof0.022�Fto0.047�F.Notethat >>>> 0.022�Fistheminimumrequiredoutputcapacitor.These >>>> capacitorstypicallyhaveanESRofabout0.1to0.5. >>>> SmallerESRcanbetolerated,howeverlargerESRcannot. >>>> Theoutputcapacitorcanbeincreasedtoimproveloadtran- >>>> sientresponse,uptoabout1�F.However,valuesabove >>>> 0.047�Fmustbetantalum.Withtantalumcapacitors,inthe >>>> 1�Frange,asmallcapacitorbetweentheoutputandthe >>>> referencepinisrequired.Thiscapacitorwilltypicallybein >>>> the50pFrange.Caremustbetakenwhenusingoutput >>>> capacitorsof1�Forlarger.Theseapplicationmustbethor- >>>> oughlytestedovertemperature,lineandload. >>>> >>>> >>>> Thanks, National. >>>> >>> I was just gonna tell you. If there was one I'd have a bumper sticker on >>> my car "LDO, no, no, no!" >>> >>> My advice to clients there is always the same, don't ever use an LDO >>> unless there is absolutely no other way (and in my 25 years on the beat >>> there always was). No matter what the marketeers sing from the trees. >> >> Unless I work off the +15, which has its own complications, I need >> this LDO voltage reference. After all, they say it's stable over - >> wait for it - a 2:1 load capacitor range! Assuming you get the ESR >> right. >> > >And nothing seems to be guaranteed. When I read page 9 in the datasheet >I am beginning to feel sick. Cute, how they say the compensation capacitor is in the "50 pF range" and how they expect *me* to compensate and extensively temperature/line/load test *their* part to ensure stability. I'm expected to do a couple thousand dollars worth of testing on their behalf. National is very sloppy as regards stability of lots of their parts. > >Why does working off of the +15V have complications? I'd need a series reference that can tolerate unregulated +15, so I'd probably have to preregulate into it... more parts. Shunt regs have mediocre TCs and would get not-very-constant current if I just use a resistor from +15. I expect that my 4-20 mA (really 0 to 25 maybe) load and uP power patterns will whack the DC-DC output voltage all over the place. There will be 12 channels per VME board, so I want low parts count and cheap parts! John
From: Jan Panteltje on 27 Mar 2010 13:41
On a sunny day (Sat, 27 Mar 2010 09:51:08 -0700) it happened John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in <3kdsq5ld9vp8ohgmg96apb3m3radmho5l7(a)4ax.com>: >>Or simply a LM317, I use LM317 for everything, very stable output at low loads. > >The TC would be terrible, espacially as it self-heats. I want overall >channel accuracy of at least 0.1%, and there are other errors to deal >with, so I can't use a rotten reference. > >John I did say this before, maybe nobody believes it, but I had the LM317 on the fluke with no load (or light load actually), and heated it up with a soldering iron, the display did not change. |